根据不同的条件查询不同的表 sql

/*
a表 a.user_id  a.name
b表 b.user_id  b.name
c表 c.user_id  c.user_type

当c表的 c.user_type = "a" 时 它显示 a表的 a.name
当c表的 c.user_type = "b" 时 它显示 b表的 b.name
a,b,c表中的 a.user_id = c.user_id,b.user_id = c.user_id

*/

if object_id('ta')is not null drop table ta
go
create TABLE ta([user_ID] int,name varchar(50))
INSERT INTO ta select
1,'a1' union all select
2,'a2'

if object_id('tb')is not null drop table tb
go
create TABLE tb([user_ID] int,name varchar(50))
INSERT INTO tb select
1,'b1' union all select
2,'b2'

if object_id('tc')is not null drop table tc
go
create TABLE tc([user_ID] int,user_type varchar(50))
INSERT INTO tc select
1,'a' union all select
2,'b'

select * from ta
select * from tb
select * from tc

--方法一

select c.[user_id],name=case user_type when 'a' then a.name  when 'b' then b.name end
from tc c,ta a,tb b
where a.[user_id]=c.[user_id] and b.[user_id]=c.[user_id] 

--方法二

SELECT     tc.user_ID, CASE user_type WHEN 'a' THEN ta.name WHEN 'b' THEN tb.name END AS 输出
FROM         tc INNER JOIN
                      ta ON tc.user_ID = ta.user_ID INNER JOIN
                      tb ON tc.user_ID = tb.user_ID


/*

user_ID     name
----------- --------------------------------------------------
1           a1
2           a2


user_ID     name
----------- --------------------------------------------------
1           b1
2           b2


user_ID     user_type
----------- --------------------------------------------------
1           a
2           b


user_id     输出
----------- --------------------------------------------------
1           a1
2           b2



*/